MDI Game Hook and Hackle Gauge.
A hackle gauge allows the fly tier to measure and select the correct size of dry fly hackle. To use the gauge, slide the guage down the stem of your vise through the center hole.
Next select a feather and wrap it around the peg on the gauge.
A series of rings indicate the size of the hackle. Hint you don't have to remove feathers to use the gauge...simply hold the gauge in your hand and turn the feather.